This invention relates to vibrating systems and to a tuned energy redistribution system for use in a vibrating system to precisely control the amplitude of vibrations in the vibrating system.
It is a problem in the field of vibrating systems to control the amplitude of vibrations that are present in such a system. One such vibrating system consists of a mass of predefined shape and extent that responds to a series of impulses by vibrating as a function of the characteristics of the mass, the frequency and magnitude of the impulses, and the location of the site at which the impulses are imparted. The computation of the characteristic response of the mass can be difficult, especially in the presence of random impulses. There are many such instances of vibrating systems and problems occurring in vibrating systems, such as: axle hop on a motor vehicle caused by a corduroy road; or axle trounce in a motor vehicle caused by extreme acceleration where the springs wind up, the axle and tires rapidly rotate, followed by the tires regaining traction, and the process repeats; or in a steel mill where extruded semi-liquid steel vibrates as it is output from the furnace causing irregularities in the steel; or in an air-conditioner compressor vibration during start up. These systems have excess vibration at a particular frequency and in a particular direction.
This undesirable vibration problem is particularly significant in vibration test systems which are used to ensure that the conditions that a product encounters during shipping, installation and use do not cause it to fail in its operation. One reason for this problem is that it is impossible to reproduce these real world conditions in a laboratory environment. The laboratory equipment that is used for testing include vibration systems, commonly known as shaker tables, that emulate the vibration conditions encountered by the product. Unfortunately, these systems are not presently able to be selectively activated at a precisely defined vibration amplitude and frequency response. It is also common to locate the shaker table within a thermal chamber to incorporate thermal cycling of the product into the vibration test.
The basic shaker table includes a platform upon which the product is mounted. The platform is supported on flexible supports that permit the vibration of the table freely in all directions, independent of the environment. The shaker table generates vibration in six axes by providing either pneumatically driven or mechanically driven actuators, termed exciters or vibrators, that produce an impact to initiate the vibrations. The platform couples the vibrations from the actuators to the product. The typical actuator is an impact device that produces forces of high magnitude but very short duration, typically driven by air pressure. There are two effects that result from this input: the repeated impacts generate a line spectrum (equally spaced lines) in the spectral density domain, the shaker table is set into a quasi-resonance condition and all of its modes of vibration are excited. As a result, the spectral density of the shakers is not uniform and can vary by six or more decades. These variations are unacceptable for highly accelerated testing or for simulation applications.
The physical properties of the shaker table components cause the shaker table to respond to the different frequencies in the impact spectrum in different ways. The physical properties of the shaker table components typically resonate with certain vibration frequencies and suppress other vibration frequencies to result in selected modes of vibration. For example, resonation results in the vibrations remaining for a relatively long time compared to the duration of the input pulse, while suppression results in the quenching of the vibration in a relatively short time. The modes of vibration of the shaker table which are excited are also a function of the location, orientation and nature of the actuators as well as the dimensions and properties of the platform. Thus, by designing the shaker table to have relatively low resonant frequencies, the spectral response of the system can be shifted to fill up the low frequency end of the spectrum, but there is a tendency to have significant variation in spectral density.
This shaker table architecture is well known and the great difficulty facing the test engineers in this field is the implementation of the shaker table in a manner to precisely produce the desired vibration conditions in terms of the presence of selected vibration frequencies and regulation of their magnitude. There are obviously numerous variables, each of which affects the magnitude and frequency of the vibrations that are produced. These variables include but are not limited to: number of actuators, actuator placement, actuator characteristics, frequency of actuator operation, physical coupling of the actuator to the shaker table platform, coupling of the product to the shaker table platform, damping elements included in the shaker table, dimensions of the shaker table, shaker table implementation, including materials and intervening structures. A further complicating factor is that these variables can also be interactive, in that the variation of one variable can impact the effects produced by another variable. Thus, the design of a shaker table is a non-trivial task and typically represents a compromise that produces a crude emulation of the desired vibration characteristics. The quest for accuracy in this field is ongoing and has been relatively unsuccessful to date.
Thus, while there exist numerous variations of shaker tables, each implementation presents limitations that prevent the test engineer from effecting precise control over the vibration frequencies and magnitudes to thereby precisely emulate the environment that the product under test will encounter or the environment desired for simulation or stimulation.
The above described problems are solved and a technical advance achieved by the present tuned energy redistribution system for a vibrating system to precisely control the amplitude of vibrations in a vibrating system. The vibrating system typically has a resonant frequency or other vibration frequency modes that are either undesirable or of excessive amplitude. The present tuned energy redistribution system functions to redistribute the vibrational energy from these undesirable frequencies to other selected frequencies, such as by spreading the vibration frequencies out over a wide band of frequencies. The present tuned energy redistribution system functions in any vibrating system and is of particular significance in the field of vibration test systems.
The present tuned energy redistribution enables a test engineer in the field of vibration test systems to precisely implement a vibration environment that is applicable to a wide range of applications and performance characteristics. The tuned energy redistribution system comprises vibration input elements and/or vibration shaping elements that collectively function to enable the user to program the frequency and magnitude of the vibrations that are produced by the vibration test system. This energy redistribution is accomplished by providing tuned absorbers, consisting of a vibrating mass and a vibration stop, which shape the frequency response of the vibrating system by responding to frequencies mostly near their natural frequency of vibration and then, when the response displacement is sufficient, provide a vibration input to the system by impacting the element in the vibrating system to which they are connected via the vibration stop, causing broad band vibration to be generated in the system by the series of impacts.
For a vibration test system which produces random vibrations to test the vibration resilience of products mounted on a shaker table, the tuned energy redistribution system is tuned to a resonant frequency that matches that of the shaker table/product under test or any other frequency where the spectral density is too high. The tuned energy redistribution system absorbs energy at its own resonant frequency and the response builds up until the vibrating or rotating mass within the tuned energy redistribution system hits the stop and an impulse is given to the shaker table. A single impulse generates an infinite series of line spectra in the frequency domain as defined by a Fourier analysis. An irregularly timed series of impulses generates a series of infinite series of line spectra in the frequency domain. Thus, energy is extracted from the system by the tuned energy redistribution system at the resonant frequency of the energy redistribution system and then redistributed over a very broad bandwidth which is determined by the stiffness of the programmer on the mass and/or stop. A very stiff programmer generates a very broad spectrum and a very soft programmer generates a narrow spectrum. Since pneumatic impactors are very weak in low frequencies, a resonance in the shaker table can be redistributed to the low frequencies by the use of a soft programmer, while a stiff programmer generates a very broad bandwidth of spectral lines. Thus, the tuned energy redistribution system can reduce resonances and fill in the low frequency end of the spectrum and, by design, the high frequency end of the spectrum can also be filled.